1. Field of the Invention
The present invention concerns a method for spatially-dependent (location-dependent) adjustment of system parameters of a magnetic resonance apparatus given continuous movement of an examination subject through a measurement volume of the magnetic resonance apparatus.
The invention likewise concerns a computer-readable medium encoded with programming instructions as well as a magnetic resonance apparatus for implementation of the method.
2. Description of the Prior Art
Various system parameters must be known a priori for the successful implementation of an imaging magnetic resonance (MR) examination. Some of these parameters are defined only once (normally during the initial operation of an MR apparatus) and are then available unchanged for all further measurements with the MR apparatus. Examples for such system parameters are the non-linearity of the radio-frequency system (RF system) of the MR apparatus and the inherent delay of the gradient system of the MR apparatus due to eddy currents.
Other system parameters are dependent on, for example, the physiology of a patient or examination subject and/or on the position of the patient in the MR apparatus during the examination. These system parameters must therefore be defined individually by adjustment measurements before each imaging or spectroscopic measurement. These examination subject-dependent system parameters describe, for example, the field inhomogeneities caused in the MR apparatus by the patient and the load situation of the RF system. A few important examples for such examination subject-dependent system parameters are the resonance frequency of the proton nuclei in the examination subject and the reference amplitude of a normalized RF pulse, for example a 180° pulse.
Such adjustment measurements have conventionally been iteratively implemented on a stationary examination subject. An iterative procedure is necessary because the desired adjustment parameters can be determined by a single measurement value only for a “physically ideal” MR apparatus. Real MR apparatuses, in particular at higher field strengths, such as as of 3 Tesla (for example), exhibit distinct deviations from the ideal behavior that can only be compensated by iterative methods.
Therefore the effect achieved with the employed system parameters is an iterative convergence on a target value for a precise adjustment. For example, a measurement is implemented with a start value for the reference amplitude of a normalized RF pulse and the achieved flip angle is determined. Assuming an ideal, linear relationship between the reference amplitude and the flip angle, a reference amplitude is calculated that should excite the desired target flip angle. This is repeated until the achieved flip angle is located within a convergence range around the target flip angle, i.e. until the achieved flip angle has sufficiently converged on the target flip angle.
A further example is the adjustment of a radiated RF band such that the resonance frequency of the proton nuclei in the examination subject represents the center frequency of the RF band. Further system parameters to be iteratively adjusted and methods for adjustment are known for adjustment measurements implemented while stationary.
Since it cannot be predicted a priori how many iterations will be required in order to converge the effect on the desired value, no advance conclusion as to the duration of a converged adjustment measurement can be made.
This means that such iterative methods cannot be used when only a fixed time duration is available for an adjustment measurement, or if only a single iteration is possible due to technical boundary conditions. Both conditions occur in adjustment measurements with continuous table movement. In that situation, system parameters must be determined not only at one stationary position but also along an entire range of positions of a patient or examination subject relative to the MR apparatus without influencing the continuous movement of the table.
A method in which a low-resolution calibration measurement is implemented, in which method the examination region is shifted continuously through the acquisition region of an MR apparatus, is described in DE 10 2005 019 859 A1. Among other things, technical information for adjustment of the MR apparatus is obtained from acquired magnetic resonance signals dependent on the position of the patient bed. Measurement protocol parameters are generated from the acquired technical information, from which measurement protocol parameters a measurement protocol is generated for a final, high-resolution magnetic resonance examination.
The absence iterations limits the precision of the attained measurement protocol parameters (adjustment values for system parameters) since the deviations of a real MR apparatus from the ideal behavior that are described above are not taken into account.